A subscriber identity module (SIM) card is also called a smart card or a subscriber identification card. The SIM card stores content such as information about a digital mobile phone subscriber, an encrypted key, and a telephone directory of the subscriber in a computer chip. The SIM card is a chip having a microprocessor and has five modules inside: a central processing unit (CPU), a program storage, a work storage, a data storage, and a serial communications unit; and the five modules are integrated in an integrated circuit. The SIM card mainly has the following functions.